Carbodiimides can be obtained by a very simple reaction from isocyanates even at room temperature according to the basic procedure described in German Pat. No. 1,130,594, using phospholine oxides as catalysts. Commercially, the most important and effective catalysts which are capable of effecting very rapid carbodiimidization of aromatic monoisocyanates and polyisocyanates even at room temperature and of converting less reactive aliphatic and cycloaliphatic monoisocyanates and polyisocyanates into carbodiimides at temperatures upwards of about 150.degree. C. are those of the general formulae ##STR1## in which R and R' represents aromatic or aliphatic hydrocarbon groups having from 1 to 14, and preferably 1 to 4 carbon atoms and R' may also represent a hydrogen atom.
Catalysts of this kind have already been used industrially for the production of polycarbodiimide foams [relative to compounds (1c) see J. org. Chemistry 32, 4066 (1967)].
Experience has shown that the formation of carbodiimides taking place in a homogeneous phase with the aid of the aforesaid readily soluble catalysts cannot be stopped at the stage necessary to obtain high quality isocyanate-containing carbodiimides or polycarbodiimides which are stable in storage. It is likewise not possible to prepare stable solutions of diisocyanato-carbodiimides or .alpha.,.omega.-diisocyanato-bis-carbodiimides or .alpha.,.omega.-diisocyanato-tris-carbodiimides or the isocyanato-uretoneimines (obtained from these products by the reaction between carbodiimide and isocyanate groups) such as those corresponding to the formula ##STR2## in excess monomeric monoisocyanates or polyisocyanates. Carbodiimidization taking place under the influence of catalytically highly active soluble phospholine oxides is practically impossible to stop with inactivating agents such as phosphorus oxychloride, zinc chloride, dimethyl carbamic acid chloride, benzoyl chloride, hydrochloric acid, boron trifluoride, alkylating agents and the like. Consequently, inferior high molecular weight insoluble products are produced. Because of the progressive (although slow) formation of carbodiimides, a high carbon dioxide pressure soon builds up inside closed vessels, which may cause serious accidents.